The purpose of this project is to gain a better understanding of the genetic fine structure of the immune response (I) region of the mouse major histocompatibility complex (MHC), and to investigate some of its gene products. The precipitation behavior of Ia specificities in F1 hybrids will be examined. Several new Ia specificities will be analyzed with attempts to predict their map position by coprecipitation methods. The heterogeneous gel electrophoresis patterns found for Ia antigens will be analyzed to try to determine whether the multiple peaks observed reflect true molecular complexity.